
This is actually a pretty cool question, but I think by the time of meal testers/wine tasters, ~medieval period~ people would know the symptoms of an allergic reaction, swelling, hives, closure of the throat rather than most poisons causing foaming from the mouth. If I had to ballpark maybe under 20 false executions although that’s a rough guess rather than a factual statement.
That’s a great question! Allergies themselves weren’t even well understood, and the first non-ambiguous descriptions and documented fatalities didn’t spring up until the early 1900s or so— and has been becoming much more common since the past few hundred years. While not an impossibility, the chances don’t seem to be all that high for it to have happened. The role of “food tester” was usually just in relation to the crown’s health in general…
And it’s unlikely a fast-acting poison would be used for food when slower-acting poisons were well known across multiple cultures. The food testers were also often the ones preparing the food, and it was kind of a means of going down with the ship you steered into a rock if something happened, and this was an extra layer of security against poisoning.
Obviously, if a tester was fine and the king was clearly being poisoned, they’re clearly suspect #1. If both got sick, showing similar symptoms, then it was legal proof someone managed to poison it. Arsenic was often a “good” poison to go with. Slow enough to get away with it and effectively undetectable.